With the increasing popularity of wide area networks, such as the Internet or the World Wide Web, network growth and traffic have exploded in recent years. Network users continue to demand faster networks, and as network demands continue to increase, existing network infrastructures and technologies are reaching their limits.
An alternative to existing hardwire or fiber network solutions, which suffer from limited capacity or exponentially increasing construction costs in “the last mile” of the communication system, is the use of wireless optical telecommunications technology. Wireless optical telecommunications utilize beams of light, such as lasers, as optical communication signals, and therefore do not require the routing of cables or fibers between locations. Data, or other information, is encoded into a beam of light, and then transmitted through free space from a transmitter to a receiver.
For point-to-point free-space laser communications, the use of narrow optical beams provides several advantages, including data security, high customer density, and high directivity. High directivity makes the achievement of high data rates and high link availability easier, due to higher signal levels at a receiver. In order to take full advantage of this directivity, some form of tracking is often necessary to keep the antennas of the transmitter and of the receiver properly pointed at one another. For example, a transmitted optical beam with a 1-mrad divergence has a spot diameter at the receiver of about 1 m at a 1-km range. Also, due to the small size of high-bandwidth, high-sensitivity photodetectors, the receiver field of view is typically less than 1 mrad. Thus, movement of the transmitter or receiver by even a small fraction of the divergence (or field of view) could compromise the link unless some form of active tracking is employed.
Charge coupled device (“CCD”) arrays, quadrant cell optical detectors, or lateral effects cells (“LECs”) are among the devices that can detect receiver pointing errors in a tracking system, and hereinafter are referred to as tracking detectors. Note that these tracking detectors may incorporate one or more methods of internal amplification to enhance sensitivity as in avalanche photodiodes or micro-channel plate photomultipliers. In any case, an electrically controllable steering mirror, gimbal, or other steering device may be used to maximize an optical signal (e.g., light) directed at a high speed detector, based on information provided by the tracking detector. This is possible since optical paths for tracking and communication are co-aligned at the time of manufacture, and the nature of a tracking signal for a perfectly aligned signal is known. CCD tracking is very sensitive, offers potentially more immunity to solar glint than simpler detectors because of the ability to ignore glint “features” on the CCD array through image processing, and is in general, a well-proven tracking method. However, at certain communication wavelengths, a tracking beam is often necessary that is separate from the communication beam and has a different wavelength that is within the spectral sensitivity band of CCD detection systems. Such separate wavelength tracking beams, often referred to as “beacons,” may be used with their own set of transmit and receive optics, thereby requiring the use of additional hardware. Furthermore, designs using separate beacon and communication optical transmitters require more time in manufacturing because of the need to co-align the two optical transmitters. Such separate transmitter paths are also more susceptible to misalignments due to mechanical shock and/or thermal stresses.
In cases where the tracking function is performed with the communication beam, a majority of the received optical signal is typically directed to the high-speed detector for the communications channel, while a small portion (e.g., 10 percent) is split off or directed to the tracking detector. For an aligned optical system using a quad cell based tracking sensor, an equal signal in all four quadrants will normally indicate that the steering mechanism has optimally directed the optical communication signal onto the high speed detector, and where there is deviation from this alignment, the steering mechanism will direct the optical signal back to this optimum equilibrium.
One method of signal detection via a tracking detector utilizes a low frequency tone superimposed on a data communication signal which can be recovered using a variety of methods in the receive electronics. An example of such a method is described in detail in commonly-assigned U.S. patent application Ser. No. 09/627,819, issued as U.S. Pat. No. 6,483,621 entitled METHOD AND APPARATUS FOR TONE TRACKING IN WIRELESS OPTICAL COMMUNICATION SYSTEMS, filed Jul. 28, 2000. This method uses a tone (e.g., 20 kHz) superimposed on a data communication signal and having a small modulation depth or occupying a distinct spectral band as compared with the primary digital or modulated data communication signal. The modulation depth of the 20 kHz tone may be as little as a few percent of the amplitude of an on-off keyed (“OOK”) signal used to convey digital information, so as not to adversely impact the data communication channel sensitivity. The advantage of tone modulation detection is an enhanced sensitivity gained via use of a narrow-band electronic filter or lock-in detector that will eliminate wide-band electronic noise. In addition, tone modulation allows the tracking system to isolate a modulated tracking signal in the presence of background light that is not modulated.
A unique tracking problem arises during inclement weather conditions, such as fog or the like, in which the “line of sight” between a pair of free-space optical terminals becomes obstructed to such an extent that the communication and/or tracking signals may be lost due to the attenuation of the signal between terminals. In situations in which the inclement weather condition persists for an extended period of time, the alignment between the pair of terminals may begin to drift. This drift may ultimately lead to a misalignment of the terminals, thereby necessitating a time-consuming re-acquisition sequence following clearing of the inclement weather condition before communications between the terminals may resume, resulting in extended delays and protracted service interruptions. In some circumstances, lingering effects of the inclement weather condition, such as fog, may also hamper the re-acquisition sequence.